some years ago, I had a friend whose son built a model glider and strapped a small 'baby-bee' two stroke engine to it. It was excellent as the engine had about a minute of fuel which was just enough to to get it a decent height and let it glide down.
The usual method was to use a big rubber band which wasn't great 'cause if the plane wasn't perfectly trimmed, it would dive to one side due to the sudden acceleration.